1. Field of the Invention
This invention relates to a disc reproducing device for detecting the position of a playback head along the radius of a disc for transferring the head to a targeted radial position on the disc.
2. Description of the Related Art
There are a variety of disc-shaped recording mediums. Of these, a replay-only optical disc such as a so-called compact disc, and a recordable optical disc capable of not only reproducing information signals but also recording and erasing the information signals, are known extensively.
With a replay-only optical disc, patterns of crests and recesses, that is phase pits, are formed in a spirally extending track or in concentric tracks on one surface of the disc.
The recordable optical discs may be exemplified by an optical disc formed of a phase transition type optical recording material and a magneto-optical disc formed of a perpendicular magnetic recording material.
The magneto-optical disc is made up of a disc substrate of a synthetic resin material, such as light-transmitting polycarbonate or PMMA, having a guide groove formed on its surface for guiding the light beam, a recording layer of the perpendicular magnetic recording material, such as Te, Fe or Co, formed for covering the guide groove, and a protective layer formed for covering the recording layer.
For reproducing the replay-only optical disc, as illustrative of the above-mentioned optical discs, a light beam from a laser light source, converged by an objective lens, is illuminated from the disc substrate side. The information signals recorded on the replay-only optical disc can be obtained by exploiting diffraction of the light beam reflected by the reflective film of the optical disc by the phase pits.
With the latter recordable optical disc, in particular the magneto-optical disc, a light beam from a laser light source, converged by an objective lens, is illuminated from the disc substrate side, as in the case of the above-described replay-only optical disc, and the Kerr rotation angle of the light beam reflected by the recording layer of the optical disc is detected for producing the playback signals in the information signals recorded on the disc.
Meanwhile, the disc management information, such as the so-called table-of-contents (TOC) information, is recorded in a pre-set position, for example the radially innermost area, of the optical disc, such that, directly after loading the disc, this disc management information usually needs to be read.
The following two methods are known as method for reading out the disc management information.
The first method resides in moving the playback head, such as an optical plate, towards the radially inner area of the disc, detecting the head position in the radially innermost area by a limit switch provided in such radially innermost area, feeding the head to a disc management information recording area (TOC area) with the detected position as a reference and adjusting the focusing for reading out the disc management information.
The second method resides in extracting from the playback signal a pre-recorded address (absolute address) along the disc radius of the position assumed by the optical pickup on starting the reproduction and calculating the amount of head movement based on the difference of the extracted address and the address of the TOC area.
The above-mentioned first method is in need of a limit switch and an increased number of components and assembling operations. In addition, the switch mounting accuracy is required, while the switch mounting space is increased.
With the second method, the following sequence of operations need to be carried out before reading out the playback signal from the disc.
First, on starting the reproduction, focusing servo is applied at an area where the optical pickup is located, at the same time that a kick voltage is applied across a spindle motor for rotating the disc.
With the focusing servo thus applied, a tracking-on state is achieved, after which the disc is accelerated or decelerated by speed control for realizing a higher probability of occurrence of the synchronization signal (11T) extracted from the playback signal reproduced from the disc.
In the speed control, the period of the synchronization signal extracted from the playback signal from the disc is compared to the length (11T) of the reference synchronization signal for performing acceleration/deceleration control so that the probability of coincidence of the two will be higher.
The state of the acceleration/deceleration control only is termed a rough servo state.
By carrying out the rough servo, it becomes possible to approach to the rotational speed corresponding to the optical pickup position on starting the reproduction.
By executing both the speed control and the phase control in this state, it becomes possible to capture the desired rotational speed. Under this condition, the phase-locked loop (PLL) is locked. The above-mentioned address can first be read in this locked state of the PLL. In particular, if the optical pickup is positioned in the vicinity of the outer rim of the disc at the time of starting the reproduction, such that the TOC area on the inner rim side of the disc is accessed after reading out the address, the time of at least 3 to 4 seconds elapses.